Particulate emissions for gasoline engines are being subject to regulations, including the upcoming Euro 6 (2014) standards. In particular, certain gasoline direct injection (GDI) engines have been developed whose operating regimes result in the formation of fine particulates. Existing aftertreatment systems for gasoline engines are not suitable for achieving the proposed particulate matter standard. In contrast to particulates generated by diesel lean burning engines, the particulates generated by gasoline engines, such as GDI engines, tend to be finer and in lesser quantities. This is due to the different combustion conditions of a diesel engine as compared to a gasoline engine. For example, gasoline engines run at a higher temperature than diesel engines. Also, hydrocarbon components are different in the emissions of gasoline engines as compared to diesel engines.
Emission standards for unburned hydrocarbons, carbon monoxide and nitrogen oxide contaminants continue to become more stringent. In order to meet such standards, catalytic converters containing a three-way conversion (TWC) catalyst are located in the exhaust gas line of internal combustion engines. Such catalysts promote the oxidation by oxygen in the exhaust gas stream of unburned hydrocarbons and carbon monoxide as well as the reduction of nitrogen oxides to nitrogen.
A catalyzed particulate trap comprising a TWC catalyst coated onto or within a particulate trap is provided in U.S. Patent Application Pub. No. 2009/0193796 (Wei). The TWC catalyst can be coated on an inlet side, an outlet side, or both of the filter.
Backpressure and volume constraints exhaust systems can limit the ability to add additional treatment components. In some GDI emissions systems, two or more TWC catalyst composites in combination with NOx traps and SCR catalysts are needed to achieve emissions standards. It is a challenge for such systems to accommodate any additional bricks or canisters along the exhaust pipe.
As particulate standards become more stringent, however, there is a need to provide particulate trapping functionality without unduly crowding the exhaust pipe and increasing backpressure. Moreover, HC, NOx, and CO conversions continue to be of interest. Certain filter technology has relatively small pores and/or smaller porosity intended to capture fine particulate matter, but such filters generally cannot accommodate sufficient catalyst loading to meet HC, NOx, and CO conversion requirements.
There is a continuing need to provide a catalyzed filter that provides sufficient TWC in conjunction with an efficient filter without unduly increasing backpressure so that regulated HC, NOx, and CO conversions can be achieved while meeting particulate matter emissions.